|Publication number||US7173848 B2|
|Application number||US 11/048,377|
|Publication date||Feb 6, 2007|
|Filing date||Feb 1, 2005|
|Priority date||Feb 1, 2005|
|Also published as||US20060171199|
|Publication number||048377, 11048377, US 7173848 B2, US 7173848B2, US-B2-7173848, US7173848 B2, US7173848B2|
|Original Assignee||Meglabs, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (15), Non-Patent Citations (2), Referenced by (16), Classifications (9), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
1. Field of the Invention
This invention relates generally to magnetic random access memory (MRAM) and more particularly to MRAM with units or stacks of multiple memory cells for producing more than two logic states.
2. Description of the Related Art
MRAM with magnetic tunnel junction (MTJ) memory cells has been proposed for nonvolatile memory, as described in U.S. Pat. No. 5,640,343 and by Reohr et al., “Memories of Tomorrow”, IEEE CIRCUITS & DEVICES MAGAZINE, September 2002, pp. 17–27. In these devices the MTJ cells are arranged as an array in a single layer (the X-Y plane) on a semiconductor substrate. In one type of architecture, called a 1T1MTJ MRAM (one transistor and one MTJ), each MTJ cell is located between a bit line and a transistor, with the word lines located beneath the MTJ cells. In another type of architecture, called a cross-point (XPC) MRAM, the MTJ cells are located directly between the bit and word lines.
In both MRAM architectures, a selected MTJ cell is programmed or “written”, i.e., its magnetic state or +/−X magnetization direction is switched, by write currents passing in X and Y directions through the bit and word lines (the write lines) located above and below the selected MTJ cell. The write currents generate orthogonal magnetic fields in the X and Y directions that switch the magnetization direction of the selected MTJ cell. The typical writing scheme is a “half-select” scheme, where each of the bit and word lines generates half the required write field for switching the selected MTJ cell. However, the energized bit and word lines reduce the magnetic reversal energy barrier in the other cells along their respective bit and word lines. This makes these “half-selected” cells more susceptible to having their magnetic states switched when the selected cell is written.
MRAM with units or stacks of multiple memory cells located between the write lines to produce more than two magnetic states, and thus more than two logic states, have been proposed. Examples of this type of MRAM are described in U.S. Pat. Nos. 5,930,164; 6,169,689 B1; 6,590,806 B1; and 6,801,451 B2; and in Published Patent Application US 2002/0036331 A1. In the prior art MRAM with multiple-memory-cell stacks, the write currents must be kept within relatively narrow margins to avoid writing cells in non-selected stacks. In addition, writing to the cells in a selected stack requires both a series of write pulses and write pulses with different current values, which increases both the complexity of the write circuitry and the time to write.
What is needed is an MRAM with multiple-memory-cell stacks that has increased write-current margins, less complex write circuitry and reduced write time.
The invention is an MRAM with memory units or stacks arranged in the X-Y plane on the MRAM substrate with each memory stack having two memory cells stacked along the Z axis. Each stack is located at an intersection region between the two orthogonal write lines. The first cell in the stack has its free layer easy axis of magnetization aligned parallel to either the X or Y axis and the second cell in the stack has its free layer easy axis of magnetization aligned substantially orthogonal to the easy axis of magnetization of the free layer of the first cell. The two cells in a stack are magnetically separated by a nonmagnetic separation layer.
Both cells are written at the same time by the application of write-current pulses of equal magnitude along orthogonal write lines. Each of the four possible magnetic states is selected by selecting the direction of write current.
The easy axis of magnetization of the free layer in each cell is the axis of anisotropy that can be induced by the shape of the cell or by the deposition process, such as deposition in an applied magnetic field or at an angle of incidence. If the anisotropy is induced during deposition, then the cells can have the same shape and matching perimeters, such as a circular shape. This allows the cells to be fabricated with the same lithographic patterning steps and permits the stacks to be packed closer together in the X-Y plane.
Each memory cell has an electrical resistance difference ΔR between the parallel and antiparallel alignment of its free and pinned layer magnetization directions. In one embodiment, the ΔR values of the two cells in a stack are different, so that each of the four possible magnetic states produced by the stack can be read by measuring the resistance across the stack. If the cells are magnetic tunnel junction (MTJ) cells, the ΔR values of the cells can be made different by fabricating the MTJ cells to have different tunnel barrier thicknesses.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.
The magnetic state of the MTJ cell is written, i.e., the magnetization direction 12 of free layer 10 is switched between +/−X directions, by write currents passing in X and Y directions through write lines W1, W2, respectively. The write currents generate orthogonal magnetic fields in the X and Y directions that switch the magnetization direction of the selected MTJ cell. The typical writing scheme is a “half-select” scheme, where each of the write lines generates half the required write field for switching the selected MTJ cell. The magnetic state of the MTJ cell is read, i.e., the direction 12 of the free layer magnetization relative to the direction 22 of the pinned layer magnetization is detected, by turning on the transistor and measuring the resistance with the read circuit when a sense current Is flows through the MTJ cell.
MRAM with multiple memory cells located between the write lines to produce more than two magnetic states, and thus more than two logic states, have been proposed. An example of a memory unit or stack of two memory cells producing four bit states is described in U.S. Pat. No. 5,930,164 and shown in
A problem arises in an MRAM that has memory units of multiple stacked memory cells because as the units become densely packed in the X-Y plane the “half-selected” unit may be switched when a nearby adjacent unit is written. To prevent the half-selected units from switching, the maximum X and Y direction write fields allowed for writing the HACs is capped at the minimum anisotropy (Hk) of the LACs sharing the write lines. In addition, the switching field for the LACs has to be kept low enough to not switch the weakest HAC sharing the write lines. As a result of this half-select constraint, the write-current margin is severely degraded. Even assuming the unlikely case where all cells are identical and thus have an identical switching astroid, the write-current window is reduced by about 50% as compared to the conventional single-memory-cell MRAM (
The invention is an MRAM having memory units of stacked memory cells in each intersection region, but in which the memory cells in the unit have nonparallel uniaxial anisotropy orientations.
The fabrication of the MRAM with the memory units of
The layers forming both the pinned layer 230 and the free layer 210 in MTJ cell 2 are deposited in the presence of a magnetic field aligned with the easy axis of cell 2 (parallel to the Y axis), which defines the overall magnetization direction of cell 2, including the magnetization direction of the AF pinning layer 240. The wafer is then annealed in the presence of the magnetic field at temperatures around 200–300° C. to set and improve the temperature dependence of the exchange coupling. Next, the wafer is lithographically patterned and etched to define the shape of MTJ cell 2, and thereby form its uniaxial shape anisotropy. After the patterning, etching, and resist removal, an insulating material, such as alumina, is used to refill the areas removed by etching. This is followed by a chemical-mechanical polishing (CMP) process to planarize the wafer and expose the nonmagnetic separation layer. This completes the fabrication process for MTJ cell 2. The process for fabricating MTJ cell 1 on top of the separation layer is essentially the same as for MTJ cell 2 except that the deposition occurs in a magnetic field aligned with the easy axis of cell 1 (parallel to the X axis). This fabrication process results in MTJ cells with the pinned layers located below the tunnel barriers, but one or both of the MTJ cells can be formed with the free layer located below the tunnel barrier. It is preferred that the AF pinning layer 140 of cell 1 have a lower blocking temperature than that of the AF pinning layer 240 of cell 2. This provides more process flexibility in achieving the two different pinning directions for the two cells. The top electrode and write line 2 (WL2) are then patterned on top of MTJ cell 1.
An alternative fabrication process for the memory unit with multiple memory cells does not rely on shape anisotropy of the cells and thus reduces the number of process steps and manufacturing cost. It is known that uniaxial anisotropy of the ferromagnetic free layer can be defined with the applied magnetic field during the deposition. The magnitude of the anisotropy can be quite high in certain materials, such as CoFeB and CoFeHf. Also, it has been reported by Pugh et al, IBM Journal of Research & Development, Vol. 4, No. 2, p. 163 (1960), that high uniaxial anisotropy can be obtained in NiFe by controlling the angular incident angle in an ion beam deposition or evaporator system. By orienting the wafer relative to the magnetic field direction and/or the incident beam angle, the anisotropy angle can be defined by deposition rather than by lithographic patterning. In this case the layers making up the two memory cells can be deposited in a single pump-down while achieving different controlled anisotropy directions for the free layers and corresponding magnetization directions for the pinned layers. If the uniaxial anisotropy of the cells is defined by a material deposition scheme, such as angular incidence, the cells can have the same shape and matching perimeters that are aligned in the Z-direction. One example would be circular-shaped cells. The uniaxial anisotropy directions for cells 1 and 2 can thus defined by depositing the free layers for cells 1 and 2 at different angular incident angles. With this fabrication approach, both cells can be fabricated with the same lithographic patterning steps. A circular geometry also allows the memory units to be packed closer together in the X-Y plane.
In this invention the writing of a memory cell occurs by applying magnetic fields along both the easy axis and hard axis of the selected cell, with each field being less than the minimum of the anisotropy field Hk. The write circuitry is capable of generating bidirectional write-current pulses. Both memory cells are “written” at the same time. The writing method will be explained by referring to
The invention has been described with the easy axes of the free layers in the memory cells in a memory unit being aligned parallel with corresponding write lines. However, the easy axes do not need to be aligned exactly parallel with the write lines but can be only generally parallel so long as the angular spacing is close enough to assure writing occurs as explained with the switching astroid of
Each MTJ cell in the two-memory-cell unit can be a “1” or a “0”, so there are four possible magnetic states for the unit. These states are represented by (cell 1 state, cell 2 state) as (1,1), (1,0), (0,1) and (0,0). If the two cells have the same resistance difference, ΔR, between the “1” and “0” states, a conventional comparator can only detect 3 resistance levels instead of the 4 needed. However, by fabricating cell 1 and cell 2 with different MTJ materials so that the cells have different ΔR values, four distinguishable resistance levels can be detected. For example, if ΔR of cell 1 is at least twice the ΔR of cell 2 the four states can be determined from the four net resistance levels, with the highest resistance being (1,1) followed by (1,0), (0,1) and (0,0). Published patent application US20020036331A1 describes an MRAM with a memory unit of two stacked conventional MTJ cells in which the ferromagnetic layers in the cells have different coercivities so that the cells have different ΔR values. The logic state of this memory unit is read by applying a voltage across it and determining the magnitude of a sense current that flows through the memory cell, with the magnitude of the sense current being proportional to the total resistance of the two series-connected MTJ cells. Different ΔR values of the two MTJs cells can also be achieved by fabricating the cells with different tunnel barrier thicknesses. This is readily achievable because the resistance-area product (RA) for magnetic tunnel junctions can be made to span 2–3 orders of magnitude for a given barrier material. For example, while the typical MTJ cell for an MRAM has a RA of approximately 1 kΩ μm2 the typical MTJ for a magnetoresistive sensor for recording head applications has RA well below 5 Ωμm2. The advantage of making the cells have different ΔR values by varying the tunnel barrier thickness is that the magnetic properties of the free layers for the two MTJ cells can still be made nearly the same.
While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.
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|U.S. Classification||365/173, 365/171, 365/158, 365/189.04|
|Cooperative Classification||G11C11/5607, G11C11/16|
|European Classification||G11C11/56B, G11C11/16|
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